Fluoropolymer dispersions have a wide variety of end uses. For coating end uses, coating compositions containing fluoropolymer dispersion are applied to any of a variety of substrates in order to confer release, chemical and heat resistance, corrosion protection, cleanability, low flammability, and weatherability. Fluoropolymer dispersions for coating use are usually in a concentrated form and typically are stabilized with a significant quantity of a nonionic surfactant such as an alkyl phenol ethoxylate or an aliphatic alcohols ethoxylate as taught in U.S. Pat. No. 3,037,953 to Marks et al., U.S. Pat. No. 6,153,688 to Miura et al., and U.S. 2003/0130393 to Cavanaugh et al.
For some specialized fluoropolymer dispersions, the presence of nonionic surfactants such as alkyl phenol ethoxylates or aliphatic alcohols ethoxylates is not desirable. One such type of dispersion is generally referred to as “raw” dispersion (also referred to as “unstabilized” dispersion) because no surfactant other than the fluorosurfactant used in polymerization is added. In some processes, raw dispersion is coagulated to obtain coagulated fluoropolymer resin which, in the case of polytetrafluoroethylene (PTFE) fluoropolymer, is referred to as fine powder. In other processes, the aqueous fluoropolymer dispersion is mixed with other materials, often in dispersion or slurry form, such as particulate polymers, fillers, pigments, solid lubricants, etc. and then the fluoropolymer is co-coagulated together with the other material. Conventional nonionic surfactants such as alkyl phenol ethoxylates or aliphatic alcohol ethoxylates are not used in coagulation processes because they generally confer high stability to the dispersion preventing or making coagulation difficult. In some processes where coagulation does succeed, the result is an undesirable sticky or fibrillated product which is difficult to process into a finished article.
In the manufacture of fluoropolymer dispersions, anionic fluorosurfactant is typically used as a polymerization aid in the dispersion polymerization process, the anionic fluorosurfactant functioning as a non-telogenic dispersing agent. For example, an early description of this use of anionic fluorosurfactant in dispersion polymerization is found in U.S. Pat. No. 2,559,752 to Berry. Because of environmental concerns and because anionic fluorosurfactants are expensive, processes have been developed for reducing and recovering anionic fluorosurfactant from aqueous fluoropolymer dispersions.
One common method to reduce anionic fluorosurfactant is by adsorption onto an ion exchange resin as taught in U.S. Pat. No. 3,536,643 (Stryker); U.S. Pat. No. 3,882,153 (Seki et al); U.S. Pat. No. 4,282,162 (Kuhls) and U.S. Pat. No. 6,833,403 (Bladel et al.) For fluorosurfactant reduction, fluoropolymer dispersions are generally stabilized with the same nonionic surfactants that are employed for coating end uses, i.e., alkyl phenol ethoxylates or aliphatic alcohol ethoxylates. However, if it is attempted to use an anion exchange process for fluorosurfactant reduction of raw fluoropolymer dispersions, premature coagulation of the dispersion will result. Such premature coagulation of the raw dispersion occurs because the dispersion is stabilized only by the presence of anionic fluorosurfactant which is being removed in the anion exchange process.
Improved aqueous fluoropolymer dispersions are desired which are especially suitable for end use applications which produce coagulated fluoropolymer resin from the dispersion.